The invention relates to an electrotherapy apparatus of the type which exposes muscle tissue to an electrical current.
Electrotherapy apparatus are used in many ways for the treatment of muscles and nerves. Use is made of the circumstance that tissue, as a rule, is not electrically neutral but carries electrical charges in the form of ions. To influence tissue in a non-mechanical manner, electrical or magnetic fields must act on the ions and these fields produce forces. The nature of the forces occurring is determined substantially by the shape, the amplitude and the timing of the applied fields. If a tissue is exposed to a DC electrical field, ions constantly flow down on the one side of this tissue and up again on the other side. Since chemical compounds can be broken down by the removal of ions, the tissue changes its structure under the protracted action of direct current. If, however, a rapid change in the polarity of the applied field occurs, whether on the basis of a sinusoidal alternating current, or on the basis of positive or negative voltage pulses, no lasting changes of the tissue occur at certain frequencies, because the ions do not move very far from their original position. They then oscillate to some extent around this original position and cause, among other things, a warming of the tissue.
In order to achieve specific therapeutic effects it is already known to expose muscles or nerves to electrical and/or magnetic fields of given frequency and amplitude (J. Low and A. Reed: Electrotherapy Explained, Butterworth-Heinemann Ltd., 1990, pp. 28 to 42, ISBN 0-07 506 00497). A distinction is made between voltages which have a length or duration of more than 1 millisecond and currents having a length of less than 1 millisecond. Also, currents or voltages are distinguished which change gradually or abruptly. The gradually varying electrical factors are subdivided into sinusoidal, diadynamic, "Russian," interfering and high-frequency factors.
For the treatment of muscular crippling a stimulating current apparatus is known which is applied to a patient's skin by means of an electrode (DE-A-29 08 365). This stimulating current apparatus first emits a unipolar series of pulses of a first polarity for a certain time, and then reverses the polarity and emits a unipolar series of pulses of a second polarity. The direct-current average of the two series of pulses is zero. Pauses of given lengths can be inserted between the pulse series. The series of pulses are intended for the purpose of greatly reducing the skin irritation which can be caused by the formation of acid between the skin and the electrodes.
Also known is an apparatus for electrostimulation with which the milk glands of cows and other mammals are treated in order to increase milk output and the fat content of the milk, and to accelerate the milking process (DE-C-29 29 293). This apparatus has a frequency modulator and a polarity setting means whereby pulses of a certain shape can be generated. In particular, square wave pulses of given length and polarity can be produced.
To stimulate and control smooth muscles and blood vessel tissues a programmable electrical apparatus is known by which succeeding positive and negative pulses or pulse groups can be produced (EP-C-0 137 007=AT-E-47 796). The pulses in this case rise with a time constant of about 40 milliseconds and fall again with the same time constant, the pulse width amounting to about 3 ms. The adjustable amplitudes of the pulses are between +130 V and -130 V, while the periods are preferably 625 ms.
In another known apparatus for the electrostimulation of nerves and muscles pulses are produced whose shape and frequency are adapted to the characteristics of the so-called slow muscle fibers (EP-C-0 197 889=AT-E-52 927). The frequency and the duration of the pulse series can be regulated.
In addition to direct current, pulsed current and simple alternating currents, modulated alternating currents are also used for electrotherapy. One of the modulated alternating currents is especially the so-called interference current, which is also called beat current or Nemec current (Otto Steuernagel, Skripten zur Elektrotherapie, 3rd ed., 1976, Vol. II, p. 6, published by the author). A distinction is made between endogenic and exogenic interference currents. In endogenic interference currents two medium frequencies are produced outside of the human body, and then heterodyned inside the body. On the other hand, the exogenic interference currents are produced outside of the body and fed as already heterodyned currents to electrodes which are laid on the patient.
The basic idea of beat current therapy is to use two heterodyned fields deep within the body to achieve a stimulating effect and at the same time keep the current density on the skin low. The heterodyning of two frequencies not too far apart brings it about that, depending on the phasing, these two currents amplify at a certain point in time and at another point in time they cancel one another (H. Jantsch and F. Schuhfried: Niederfrequente Strome zur Diagnostik und Therapie, 1974, p. 147).
Furthermore, a heterodyne current therapy is known in which three alternating currents in the medium-frequency range of about 4 kHz are superimposed, which differ in frequency from one another only by a small amount, so that beats result from the superimposition (DE-A-30 10 716). The current curves resulting from the superimposition have the shape of sinusoidal envelope curves on both sides of the potential null line, while sine half-waves of large amplitude alternate with small amplitudes, comparable to a two-sideband amplitude modulation with suppressed carrier (cf. Meinke-Gundlach: Taschenbuch der Hochfrequentztechnik, 4th Edition, Vol. 3, Systems, 1986, O 3, FIG. 3, illustration in the time part). In the center the half-waves of the beat envelopes opposite one another at any time cancel their potential, i.e., there are no direct-current components. Nevertheless the two electrodes which are brought against a muscle to be treated are still at potential, so that an alternating current symmetrical with the null line is still flowing. The muscle is thus treated locally always in the same manner; any change in the preferred direction of treatment during a given time does not occur. A frequent result is muscular cramping, which is disagreeable to the patient.